ring oscillator and LC oscillator in analog
Ring oscillators are preferred in PLLs for clock signal generation due to their simplicity and low power consumption. However, their harmonic distortion is high and their amplitude is fixed; both characteristics are undesirable for the target applications of this PLL. LCbased oscillators have a superior linearity and phase noise performance but their required area can become prohibitively large for frequencies below 1GHz. For signal generation in the range of tenths to few hundreds of MHz, a transconductance-capacitance (OTA-C) oscillator structure offers amplitude control and low distortion in a compact implementation, and hence is chosen for this design. Figure 5 shows a block diagram of the designed differential quadrature oscillator. The oscillation frequency is determined by the ratio gmω/C.